Thinking back to Baldur's Gate 1 and 2 (not going to consider Dark Alliance in this argument) I fondly remember parties of 6. It created a sense of diversity- but you'd also likely have two of one class, allowing for those two classes to approach their character progression differently or provide different versions of that class's function on the battlefield.
The larger parties could conceivably help balance the strength and frequency of long / short rests- providing more CR for the player to work with. It allows for more of the characters Larian is spending so much time on to join up with the party- but also leaves a lot of room for deaths and replacements- of which I think and hope there will be plenty of. I will be more than sorely disappointed if the only party members available to you in the game are the ones we've met. I doubt this to be the case, but I've been disappointed before.
This also opens up more room for multiplayer- allowing 6 people to play (IMO the best size D&D group) or 2 playing 3, 3 playing 2, or even just 4 friends taking one character each and testing themselves against a higher CR combat- or maybe it auto-tunes based on current party CR.
Either way, I whole heartedly agree with OP's sentiment- and I really hope they look at the roots of Baldur's Gate and go with a 6 character party alongside a slew of other recruitable miscreants with memorable voicelines you hear 5,000 times.
